Soften gelatin in cold water; set aside. In a saucepan, combine sugar, cornstarch and salt. Blend in the milk and egg yolks; cook over low heat, stirring constantly, until thickened and bubbly, about 20-25 minutes. Remove from the heat; stir in softened gelatin until dissolved. Stir in butter and vanilla. Cover the surface of custard with plastic wrap and chill until no longer warm. Slice 3 bananas; fold into custard with whipped cream. Spoon into pie shell. Chill until set, about 4-5 hours. Shortly before serving time, place lemon juice in a small bowl and slice the remaining banana into it. melt jelly in a saucepan over low heat. Drain banana; pat dry and arrange on top of pie. Brush banana with the jelly. Sprinkle with grated lemon peel. Serve immediately.